1. Field of the Invention
The invention relates to a microwave plasma processing system which performs a processing such as etching, ashing, or thin film formation in a production of a semiconductor device.
2. Description of Related Art
In a plasma processing system, a gas discharge is performed in a vacuum vessel under a reduced pressure or a low gas pressure by introducing microwaves into the vessel, thereby generating a plasma, and the surface of a substrate which is a sample is irradiated with the generated plasma so that a processing such as etching, or thin film formation is conducted on the substrate. Since such a plasma processing system is essential for the production of a highly integrated semiconductor element or the like, research and development thereof are intensively conducted. Particularly, a microwave plasma processing system in which the generation of a plasma and the acceleration of ions in the generated plasma can be controlled independently is desired in the dry etching technique and the burying technique for thin film formation.
FIG. 1 shows a conventional microwave plasma processing system proposed by the applicant of the present application in Japanese Patent Application Laid-Open No. 5-144773(1993). The system is aimed at independently controlling the generation of a plasma and the acceleration of ions in the plasma. In the figure, numeral 11 designates a reactor which is made of a metal such as aluminum (Al), or a stainless steel. The reactor 11 is airtightly partitioned by a microwave introduction window 14. The window 14 consists of a refractory plate made of a material such as quartz glass, or Al.sub.2 O.sub.3 which allows microwaves to be transmitted therethrough, produces a small dielectric loss and has heat resistance.
In the upper portion of the reactor 11 which is partitioned by the microwave introduction window 14, disposed is a dielectric passage 17 which opposes the microwave introduction window 14 with a predetermined distance therebetween and has a size sufficient for covering the microwave introduction window 14. The dielectric passage 17 comprises a dielectric layer 17b which is made of a dielectric material producing a small dielectric loss, such as fluororesin, polystyrene, or polyethylene, and a cover 17a which is made of Al or the like and disposed on the upper face of the dielectric layer 17b. Microwaves are introduced from a microwave oscillator 19 into the dielectric passage 17 via a waveguide 18.
In the lower portion which is partitioned by the microwave introduction window 14, a reaction chamber 13 is formed. A sample stage 15 having a sample holder 15a for holding a sample S which is to be processed is disposed inside the reaction chamber 13. A high frequency power source 15b for generating a bias voltage at the surface of the sample S is connected to the sample holder 15a. The sample holder 15a is provided with a suction mechanism for sucking the sample S due to an electrostatic chuck, and a cooling mechanism for cooling the sample S due to a circulating coolant or the like.
The side wall of the reaction chamber 13 has a double-wall structure so that a flow path 12 serving as a passage for cooling water is formed inside the double-structured side wall. An inlet pipe 12a, and an outlet pipe 12b are communicated with the flow path 12. A gas supply pipe 16b through which gasses required for generating a plasma are supplied into the reaction chamber 13 is connected to the upper portion of the side wall. A gas discharge port 16a which is connected to an evacuating apparatus and through which the reaction chamber 13 (reactor 11) is evacuated to attain a vacuum is disposed in the lower wall of the reaction chamber 13.
The reactor 11, the microwave introduction window 14, the sample holder 15a, the dielectric passage 17, and so on constitute the microwave plasma processing system 10. The plasma processing area can be varied by changing the horizontal section areas of the microwave introduction window 14, and the dielectric passage 17.
Hereinafter, the case where a processing, for example, an etching is to be conducted on the surface of the sample S in the thus configured microwave plasma processing system 10 will be described.
At first, evacuation is conducted through the gas discharge port 16a to set the interior of the reaction chamber 13 to have a predetermined pressure, and reactant gasses are then supplied through the gas supply pipe 16b. Cooling water is supplied through the cooling water inlet pipe 12a to be circulated in the flow path 12, and then discharged into the cooling water outlet pipe 12b. Then the microwave oscillator 19 oscillates microwaves and the generated microwaves are introduced into the dielectric passage 17 via the waveguide 18 so that an electric field is generated below the dielectric passage 17. The generated electric field is transmitted through the microwave introduction window 14 so that a plasma is generated in the reaction chamber 13. At the same time, in order to control the anisotropy and the acceleration energy of ions in the plasma, the high frequency power source 15b applies a high frequency electric field to the sample holder 15a on which the sample S is mounted, so that a bias voltage is generated at the surface of the sample S. The bias voltage causes the ions to be perpendicularly incident upon the sample S and controls the energy of the ions incident upon the sample S, thereby conducting the etch processing or the like on the sample S.
Although, a system is proposed which is configured so that, in the microwave plasma processing system 10, an electrode made of, for example, an Al plate is in contact with the lower face of the microwave introduction window 14, or disposed at a predetermined position at the midpoint between the microwave introduction window 14 and the sample holder 15a (Japanese Patent Application Laid-Open No. 6-104098(1994)). Through holes through which the microwave is to be transmitted are formed in the electrode, and the electrode is grounded. The plasma potential generated when a high frequency electric field or a DC electric field is applied to the sample holder 15a is stabilized. Therefore, a stable bias voltage is generated at the surface of the sample S so that the directivity and the acceleration energy of ions in the plasma are further optimized.
Recently, such a method is proposed in which the side wall of the reaction chamber 13 is heated so that reaction products or decomposition products (hereinafter, referred to merely as "products") produced by the discharge energy of a plasma are suppressed from adhering to the side wall of the reaction chamber 13, thereby increasing the density of products in the vicinity of the surface of the sample S (Japanese Patent Application Laid-Open No. 4-256316(1992)). According to the method, in a process of etching a silicon oxide (SiO.sub.2) film, the SiO.sub.2 film can be etched away with a high selectivity with respect to the ground silicon (poly-Si) film.
However, the microwave plasma processing system 10 has a problem in that, even when the side wall of the reaction chamber 13 is heated, products easily adhere the side wall of the reaction chamber 13 little by little to form a deposition film, the deposition film flakes off as particles, and the particles adhere the surface of the sample S, whereby the plasma processing is adversely affected. In order to prevent such particles from being produced, the deposition film is removed away by cleaning using an oxygen plasma. However, this dry cleaning step requires much labor and produces a problem in that the efficiency of the plasma processing is lowered. Furthermore, there is a problem in that, when wafers serving as the sample S are successively subjected to a plasma processing one by one, the selectivity is gradually changed, with the result that it is difficult to always conduct an efficient plasma processing with excellent reproducibility.